The present disclosure relates to thermoelectric materials that can be used in thermoelectric devices for electric power generation, and more particularly to partially-reduced, doped oxides that have a high thermoelectric figure of merit.
Thermoelectric materials can be used to generate electricity when exposed to a temperature gradient according to the thermoelectric effect. Notably, a thermoelectric device such as a thermoelectric power generator can be used to produce electrical energy, and advantageously can operate using waste heat such as industrial waste heat generated in chemical reactors, incineration plants, iron and steel melting furnaces, and in automotive exhaust. Efficient thermoelectric devices can recover about 5% or more of the heat energy released by such industrial systems, though due to the “green nature” of the energy, lower efficiencies are also of interest. Compared to other power generators, thermoelectric power generators operate without toxic gas emission, and with longer lifetimes and lower operating and maintenance costs.
The conversion of thermal energy into electrical energy is based on the Seebeck effect, a phenomenon that describes the formation of an electrical potential in a material that is exposed to a thermal gradient. The Seebeck voltage, ΔU, also referred to as the thermopower or thermoelectric power of a material, is the induced thermoelectric voltage in response to a temperature difference across that material. The Seebeck coefficient S is defined as the limit of that thermoelectric voltage when the temperature gradient goes to zero,
  S  =      lim    ⁢                  Δ        ⁢                                  ⁢        U                    ∇                                  ⁢        T            and has units of VK−1 though typical values are in the range of microvolts per Kelvin.
A thermoelectric device typically includes two types of semiconducting material (e.g., n-type and p-type) though thermoelectric devices comprising a single thermoelectric material (either n-type or p-type) are also known. Conventionally, both n-type and p-type conductors are used to form n-type and p-type elements within a device. In a typical device, alternating n-type and p-type elements are electrically connected in series and thermally connected in parallel between electrically insulating but thermally conducting plates. Because the equilibrium concentration of carriers in a semiconductor is a function of temperature, a temperature gradient in the material causes carrier migration. The motion of charge carriers in a device comprising n-type and p-type elements will create an electric current.
For purely p-type materials that have only positive mobile charge carriers (holes), S>0. For purely n-type materials that have only negative mobile charge carriers (electrons), S<0. In practice, materials often have both positive and negative charge carriers, and the sign of S usually depends on which carrier type predominates.
The maximum efficiency of a thermoelectric material depends on the amount of heat energy provided and on materials properties such as the Seebeck coefficient, electrical conductivity and thermal conductivity. A figure of merit, ZT, can be used to evaluate the quality of thermoelectric materials. ZT is a dimensionless quantity that for small temperature differences is defined by ZT=σS2T/κ, where σ is the electric conductivity, S is the Seebeck coefficient, T is temperature, and κ is the thermal conductivity of the material. Another indicator of thermoelectric material quality is the power factor, PF=σS2.
A material with a large figure of merit will usually have a large Seebeck coefficient (found in low carrier concentration semiconductors or insulators) and a large electrical conductivity (found in high carrier concentration metals).
Good thermoelectric materials are typically heavily-doped semiconductors or semimetals with a carrier concentration of 1019 to 1021 carriers/cm3. Moreover, to ensure that the net Seebeck effect is large, there should only be a single type of carrier. Mixed n-type and p-type conduction will lead to opposing Seebeck effects and lower thermoelectric efficiency. In materials having a sufficiently large band gap, n-type and p-type carriers can be separated, and doping can be used to produce a dominant carrier type. Thus, good thermoelectric materials typically have band gaps large enough to have a large Seebeck coefficient, but small enough to have a sufficiently high electrical conductivity. The Seebeck coefficient and the electrical conductivity are inversely related parameters, however, where the electrical conductivity is proportional to the carrier density (n) while the Seebeck coefficient scales with 3n−2/3.
Further, a good thermoelectric material advantageously has a low thermal conductivity. Thermal conductivity in such materials comes from two sources. Phonons traveling through the crystal lattice transport heat and contribute to lattice thermal conductivity, and electrons (or holes) transport heat and contribute to electronic thermal conductivity.
One approach to enhancing ZT is to minimize the lattice thermal conductivity. This can be done by increasing phonon scattering, for example, by introducing heavy atoms, disorder, large unit cells, clusters, rattling atoms, grain boundaries and interfaces.
Previously-commercialized thermoelectric materials include bismuth telluride- and (Si, Ge)-based materials. The family of (Bi,Pb)2(Te,Se,S)3 materials, for example, has a figure of merit in the range of 1.0-1.2. Slightly higher values can be achieved by selective doping, and still higher values can be reached for quantum-confined structures. However, due to their lack of chemical stability and low melting point, the application of these materials is limited to relatively low temperatures (<450° C.), and even at such relatively low temperatures, they require protective surface coatings. Other known classes of thermoelectric materials such as clathrates, skutterudites and silicides also have limited applicability to elevated temperature operation.
In view of the foregoing, it would be advantageous to develop thermoelectric materials (e.g., n-type and/or p-type) capable of efficient operation at elevated temperatures. More specifically, it would be advantageous to develop environmentally-friendly, high-temperature thermoelectric materials having a high figure of merit in the medium-to-high temperature range, e.g., above 450° C.